1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for continuous data acquisition for geophysical exploration.
2. Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, which is information especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, sources in common use are essentially impulsive (e.g., compressed air is suddenly allowed to expand). One of the most used sources is airguns. An airgun produces a high amount of acoustics energy over a short time. Such a source is towed by a vessel at a certain depth along direction X. The acoustic waves from the airgun propagate in all directions. The airgun instantaneously releases large peak acoustic pressures and energy. Such a source is illustrated in FIG. 1. This figure shows a source array 104 being towed behind a vessel 101 at a shallow depth. When the source array is activated, acoustic energy is coupled into the water and transmitted into the earth, where part of the energy is partially reflected back from the ocean bottom 113 and from rock formation interfaces 112 (rock layer that has a change in acoustic impedance). Sensors or receivers 106 used to record the reflected energy include hydrophones, geophones and/or accelerometers. The receivers can be encapsulated in either fluid filled or solid streamers 105 that are also towed by vessels at shallow depth.
Currently, it is typical for one vessel to tow multiple streamers with diverters employed to ensure streamer separation by a fixed distance. In order to maintain the proper spacing between the streamers and sources, the vessel moves forward continuously, typically at a rate of about 4 knots (2 m/s). In some cases, the streamer can be controlled so that all receivers are at a common depth, or in other cases the receivers in each streamer are controlled to follow a particular depth profile.
Modern streamers are equipped with birds, compasses and GPS receiver buoys. Birds are devices equipped with fins, spaced at intervals that are in communication with the vessel to control streamer depth and transverse spatial position. Alternatively, the receivers can be stationary and positioned on the ocean floor as autonomous nodes or in an ocean bottom cable.
Depending upon the sensor type, the returning energy is recorded as a pressure, velocity or acceleration variation as a function of time at each receiver position. Combining recordings made at multiple source and receiver locations can be used to form an image of the subterranean features of the earth. Images formed from reflection seismology are useful for locating structures that are indicative of oil and/or gas reservoirs.
However, the frequency content of impulsive sources is not fully controllable, and different number, sizes and/or combinations of airgun sources are selected depending on the needs of a particular survey. In addition, the use of impulsive sources can pose certain safety and environmental concerns.
Thus, another class of sources that may be used is vibratory sources. For vibratory sources, the source signal excitation is typically a chirp (swept frequency sine wave excitation signal over a pre-determined sweep bandwidth for a predetermined time interval). The source array emits a chirp over a given sweep length as it is towed by a moving vessel. Typically, after some instrument reset period and/or listen time, the chirp is repeated to start a new recording for the new source/receiver position. Thus, a typical raw record includes both sweep and listen time. Correlation may be employed to collapse the data to produce a record that is similar to what might be obtained using an impulsive source. The technique of using a vibratory source followed by correlation to collapse the data is called Vibroseis.
An alternative to correlation is source signature deconvolution, whereby a measured source signal is used to convert the extended source signal to an impulse, which involves the performance of some form of spectral division. In source signature deconvolution, a fast Fourier transform (FFT), of a received signal and a measured source signal are taken using either uncorrelated or correlated data. A spectral quotient is formed in which the received spectrum is divided by the source frequency spectrum at each frequency. An array including the resultant spectral quotients is converted back to the time domain using an inverse Fourier transform operation (IFFT), to recover the earth impulse response.
Generally, seismic data acquired in marine surveys is superior to that collected in land surveys. Source coupling in water is much better and homogeneous than for land. On land, source coupling is much more variable than at sea because the vibrators shake on surfaces that can quickly change from sand to rocks to tree stumps, roads, mud, etc. The marine environment is generally quieter than for land surveys resulting in recordings with lower ambient noise levels.
However, there are special problems that arise in marine seismology. Because the source is located below the surface of the water, this gives rise to a surface reflection event referred to as a surface ghost. The acoustic reflection coefficient of the surface is essentially −1, so that up-going pressure waves radiated by the source undergo a polarity reversal when they reflect downward off the water's surface. These ghosts destructively and constructively interfere with the primary radiated energy from the source to produce spectral peaks and notches in the power spectrum of the radiated energy.
FIG. 2 depicts the effect of the source ghost on the power spectrum of a vertically propagating signal generated by two sources. The curve 200 corresponds to a source operating at a depth of 20 m and has notches in its spectrum at approximately 0, 37.5, 75, 112.5 and 150 Hz. For curve 202, the source is at 5 m depth and notches in its spectrum appear at 0 and 150 Hz. The curves in FIG. 2 have been normalized to their respective peak values. The surface ghost produces constructive energy to produce the curve peaks in FIG. 2.
It is also noted that at the very low end of the spectrum and below 30 Hz, the source at 20 m depth has significantly more output than the shallow source. Thus, if these ghosts are not addressed, they can lead to spectral deficiencies in the reflection data. The frequencies at which these notches occur are a function of the source depth and the ray path. Since most of the energy useful for acoustic illumination in reflection seismology is close to vertical, spectral notches produced for ray paths near vertical are of particular concern. Deficiencies in the spectral content of the radiated source energy can compromise the quality and resolution of the processed image.
Another matter of some concern for marine vibratory sources is the fact that the radiated energy is spread out over time. Because the vessel, source and receivers are moving, time and space are mathematically coupled. If the sources emit a swept frequency signal, the source spectrum changes as the source moves. Energy received will also be affected by motion. Generally, a correction for receiver motion is easier to calculate than a correction for source motion, because during a survey, the vessel moves in a straight line at constant speed and the receivers follow one another. Thus, during a sweep, one or more receivers will pass over the same position. Therefore, a simple interpolation method could be employed to combine adjacent receivers to create a virtual receiver that appears stationary.
For chirps, the lower the sweep rate, and/or as frequency is increased, the greater the resultant phase dispersion caused by Doppler shifting of the source sweep signal. In this respect, Allen (U.S. Pat. No. 6,049,507) teaches a method for correcting the source motion by sorting the data into constant dip slices by transforming the data into the F-K (frequency wave-number) domain, computing and applying the necessary motion correction to each slice and then summing the results.
Just like their land counterparts, marine vibratory sources have spectral output limits imposed upon them by system constraints. These constraints may be mechanical, for example actuator stroke may limit the amount of travel of an acoustic driver thereby limiting the maximum peak temporal low frequency content of a sweep. For marine vibrators driven by hydraulic actuators, the maximum pump flow rate may limit the driver velocity and the hydraulic supply pressure may limit the force that can be developed at high frequency. Or, as can be the case for vibratory sources driven by electromagnetic actuators, electronic components may impose acoustic output constraints at other frequencies due to voltage and/or current limits.
Recently, a number of simultaneous source acquisition methods have been disclosed primarily for use in land seismic surveys that are useful for increasing the rate at which data can be acquired, thereby reducing the amount of time required to conduct a survey. Becquey (U.S. Pat. No. 6,704,245) discloses a method for simultaneous acquisition of Vibroseis data that requires the use of maximal length binary coded sequences in combination with circular permutation.
Two schemes are disclosed. In one realization, all sources use time delayed versions of the same coded sequence, with each source array using a unique delay. Circular correlation is employed to separate the contributions of each source and then selecting the interval of interest ascribed to a particular source time lag. In an alternate implementation, unique maximal length codes are selected for each source array, and the different codes are selected to be mutually weakly correlated. Signals are simultaneously emitted into the ground and a composite record contains the superposition of the source emissions, each convolved with the earth impulse response representative of the signal path from the source through the earth and to the receiver. Circular cross-correlation of the received data with the different coded sequences is used to separate the source contributions to the composite record.
However, Becquey does not teach how to construct band-limited signals of arbitrary length that do not rely on maximal length binary codes. Further, Becquey does not describe how to modify pseudorandom sequences to better suit their implementation on real hardware.
Sallas and Gibson (U.S. Pat. No. 7,859,945, the entire content of which is incorporated herein by reference) teach a method for generating and separating simultaneous emissions from ground seismic vibrators. That method creates pseudorandom signals that are only weakly correlated over a time window of interest. These signals are simultaneously emitted into the ground by vibrators occupying different locations. The superimposed signal, after traveling through the earth, is recorded using a shared receiver line. The composite record is correlated and windowed with the various excitation signals as well as measured source signals. After transforming the windowed correlated signals into the frequency domain using FFT's, a matrix separation method is used to separate the individual source computations frequency by frequency. The resultant matrix vectors are then frequency inverse transformed, back to the time domain, thereby creating a useful source signature deconvolution scheme.
Smith (U.S. Pat. No. 6,942,059) teaches a method whereby multiple marine vibrators are deployed at different depths to form a composite source array. For each depth a unique chirp sweep or suite of sweeps are prescribed. The source contributions for each depth can be separated by virtue of the fact that they either cover different bandwidths and/or have different sweep rates and/or have frequencies that overlap at different times. The objective of Smith is two-fold: to increase productivity by covering the overall seismic bandwidth more quickly and to eliminate the source ghost and the resultant spectral notches created by surface reflections.
One practical difficulty with this approach is that it does require a high combined source output energy level that is able to accomplish its stated objective of acquiring a shot gather in the same time as is done with air guns (typically 6 s).
To help mitigate problems associated with equipment constraints, Bagaini (U.S. Pat. No. 7,327,633) describes a method that takes a low frequency constraint due to actuator stroke into account in the design of vibrator chirp sweeps. Sallas (U.S. Patent Application Publication No. 2011/0085416) provides a vibrator bandwidth extension while honoring multiple equipment and environmental constraints. Both documents address just Vibroseis acquisition when swept sine wave sweeps (chirps) are to be employed.
In seismic acquisition, it is desired to perform the survey in the shortest amount of time possible. The faster a volume of data can be acquired without significant compromise to quality, the lower the cost of data acquisition. Thus, a method that can continuously and simultaneously record data from various sources without stopping is valuable. There is no need to repeatedly start and stop recording. Furthermore, a system that allows flexibility in the way the recorded data may be parsed later, during processing, provides an approach in which shot density can be increased to improve survey spatial sampling if desired.
Thus, there is a need to provide a method for reducing an acquisition time of a seismic survey performed with a vibratory source.